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The day may come when you want to remove kittens after the mother is dead or captured by you in an attempt to control too many cats on the premises or the neighborhood. Kittens are cute, but too many can actually be harmful to you and the population of too many cats. I won't go into the dangers.
When my two mother cats were poisoned by a spiteful neighbor, their three shared kittens disobeyed orders and came out of hiding. Three days in a row, they came to me as I returned home, scrambling from under the porch where she kept them. One actually bit my little toe as hard as he could before I figured out they might be hungry. I talked to them and they went away while I returned with cat food, the hard crunchy kind. After I called them in my mama cat voice, they scrambled out on unsteady feet. I had rushed back. I wanted to return with something that they would recognize from their mothers having eaten it. I figured mama cats might have had cat food breath.
If you need to remove a mama cat and kittens, I would not recommend leaving the kittens to starve. But, I would not condemn you either. But, their might be a better way if you can follow my advice borne of experience. Besides, dead kittens under the porch might not be pleasant.
If you trap the Mama cat, you could remove it from their hearing and take her voice from their hearing--move trapped mama far away. Then, maybe the kittens would show themselves if you learn mama-cat-speak. Three little kittens, about a month old, learned to crunch cat food that first time. They were so desperately hungry it broke my heart. Where was mama? Then, I put milk on the food for ten minutes before I brought it to them, and they had an easier time. They learned to drink milk by choking on it as they walked in the bowl. They would not come out on their own or get near me unless I called them like mama after the singular toe-nipping. I did not have a live trap then.
I actually had hidden in the house the week before with the door cracked a bit and learned to call kittens from hiding like their mother did. That is probably the only reason they dared to come and chew my little toe. Sometimes, I called them just to see if I had it right. They looked around, looked puzzled, and retreated as I came near them.
The kittens having seen me and heard my voice may be the only reason they crawled over my feet for several days as I tried to enter the house. I don't know. They were desperate to get food. Their mamas were barn cats given to me as males! Since I could not see, I believed these friends who probably did not know either, but were looking to dump extra cats. Beware! I was looking for rat and mouse control around the house since I lived near a hospital dumpster. The mothers would not come to me or allow petting, but they did hunt, and I kept food for them, hoping they would stay as feral cats who just liked my yard and under-the-porch housing free from rain and probably warm from the house.
To call a kitten like mama, use a short, very soft trill that is neither high- or low-pitched. You probably will not get a kitten to come the first time. Keep practicing and trying. Sit very still.
As I fed these furtive little balls of fur, I wondered where the mamas were. I did not try to catch them or try to give them a box to live in because I thought the mamas would return. The mamas always took turns nursing. One would lie in the yard while the other mama would fetch grasshoppers for the kittens amusement and to hone hunting skills. The mama were attentive, so I could not figure out why they did not return.
The mothers were dead but I did not know that at first. There was no way I could hope for their return when I figured that out. I did not discover the bodies until a week later, trying to get into the basement, but dying on the outdoor, concrete steps.
The kittens remained feral and part of the growing feral cat population. Eventually, I caught 8 cats, one of whom had 8 kittens that were hand caught since they barely had their eyes open.
Maybe you can develop a mama-cat-voice. Listen to how mama does it if you have non-feral cats that are in the house. Women will probably be able to match the tone better. I have used this method just to prove I could lure kittens out of hiding, so I know it works. However, I did not use my newly found skill for evil. You might not be able to hand-catch a kitten, but you can lure it out so it smells food you provide, especially if Mama Cat is removed and out of hearing. If you feed your cat commercial cat food, provide that first to the kittens, even if it is not appropriate. My theory: They know mama-breath.
How to trill for the kittens. It sounds sort of like a purr. Put your tongue behind your upper teeth. Inhale. Close your mouth. Push air from your throat as you open your mouth just a little bit. Practice.
Have you ever learned how to trill like mama cats call their kittens?